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The results showed that those delivered by Caesarean section, and who therefore did not pass down the mother’s birth canal, lacked one of the major groups of gut bacteria, the Bacteroidetes, or acquired it late when compared with babies born vaginally.

In some babies born by Caesarean, Bacteroidetes did not appear until a year after birth. The total range of bacteria among babies born in theatre was also lower.

The experts said: ‘Microbial colonisation of the infant gut gastrointestinal tract is important for the postnatal development of the immune system.

The lower levels of good bacteria may put babies born by Caesarean at higher risk of allergies

‘In this study, Caesarean section-delivered infants who are not entering the birth canal of the mother either lacked or displayed a delayed colonisation of one of the major gut phylum, the Bacteroidetes.

‘The colonisation of this phylum was delayed by up to one year for some infants. The total microbiota diversity was also lower in the Caesarean section infants, probably largely as a consequence of the lack of this phylum.’

Bacteria are important for prompting the immune system to respond to triggers and to not overreact as is the case in allergies, diabetes, and inflammatory bowel disease, the experts said.

The results had nothing to do with whether the mother had received antibiotics to help prevent infection.

The gastrointestinal tract of newborn infants is considered to be sterile in the womb, the authors went on.

‘Bacteria from the environment, mainly from the mother, colonise the infant gut immediately following birth,’ they said.

Bacteroidetes do not appear to grow outside the gut and ‘hence need to be transmitted between human hosts’.